Environmental groups have poured a lot of money and energy into stopping the Keystone XL pipeline. The Obama administration has been in a long vetting process over whether to approve this project. The pipeline would carry carbon-rich oil several thousand miles from the tar sand fields of Canada South. Many environmentalists see this fight as an important test of whether the U.S. is committed to controlling carbon emissions.

And one of America's most influential scientists, Science magazine editor-in-chief, Marcia McNutt was initially on their side. She agreed that the pipeline was a bad idea.

MARCIA MCNUTT: From what I knew of the Canadian tar sands, they were a rather large emitter of CO2 compared to alternatives. And the original routing went through some fairly sensitive areas, such as breeding ground of the Sandhill Cranes.

GREENE: But Marcia McNutt has now changed her mind. It's a potential blow to environmental groups. In a Science magazine she writes that she now supports the Keystone pipeline. We asked her why she changed course.

MCNUTT: Just because there hasn't been a pipeline really did not stop the development of the Canadian tar sands.

GREENE: They were going to be developed anyway, you're saying.

MCNUTT: Yeah. In fact, they are developed anyway. Rather than putting the oil in a pipeline, they are now putting the oil on trucks and railway cars and trucks and trains actually use more fossil fuels themselves to get that crude oil to market than a pipeline.

GREENE: OK. So one message to environmentalists is that this is going to happen anyway and that doing it in pipeline might be the cleanest of the options.

MCNUTT: Not only the cleanest, but potentially safer because the pipeline is still to be permitted, environmentalists can demand the pipeline be the safest ever engineered. One of the reasons for opposing the pipeline is the emissions of greenhouse gases when the tar sands are converted to a liquid to put into the pipeline. There actually could be some concessions in exchange for approving the pipeline that could require a limit on the carbon emissions in that process.

GREENE: What about the argument from environmentalists that if you build this pipeline that guarantees that this fossil fuel is going to be taken from the ground and transported and, you know, sold for many, many, many years to come?

MCNUTT: I don't see any argument that tells me that not building the pipeline guarantees that it won't.

GREENE: I wonder, I mean this is a big battle for environmentalists. And they feel like they are in a philosophical debate about the direction of this country when it comes to energy policy. You know, what do you say to them to convince them that this is a moment to back down?

MCNUTT: Well, I'm totally sympathetic to the argument. On the other hand, I'm also a pragmatist. We need to find a funding source that allows us to invest more aggressively in solar, wind, and other non-CO2 polluting sources. Now, if you look at the cost of transporting oil in a pipeline, it is the very cheapest way to do it. If one can identify a revenue stream that would come from all of the money saved and convert that money then to a renewable energy fund that sets us on that right path, then I think the nation really wins.

GREENE: Do you think environmentalists would be open to hearing that message?

MCNUTT: I think some would. I hope many would. I think in a time of austerity in this country, why not make the polluting technology our funding source for the future?

GREENE: Marcia McNutt, thank you so much for joining us to talk about this. It's been a real pleasure. We appreciate it.